MIMO (multiple input multiple output) transmission/reception concepts or diversity is used in communication systems such as wireless communication systems in order to enhance communication quality by transmitting information to be transmitted via various (e.g. space and/or time varying) channels. Common to both concepts, MIMO and diversity, is that multiple parallel beams are needed. The expression “beam” as used herein means a spatial transmission channel.
For the sake of keeping the explanations simple, the following description focuses on diversity, while the given explanations are equally applicable to MIMO concepts.
Diversity arrangements can be categorized as open-loop diversity and closed-loop diversity concepts. Also hybrids are possible, which combine both open-loop and closed-loop approaches.
Closed-loop diversity concepts apply channel state information at the transmitter. In systems with channel reciprocity (e.g. Time Divisional Duplex, i.e. TDD) this is inherently available. The invention is thus applicable also in such systems. However, in conventional FDD systems (Frequency Divisional Duplex) channel reciprocity does not hold, and a feedback channel conveying channel-related information from the receiver to the transmitter is needed to thereby perform a feedback control of the diversity transmission.
The closed-loop techniques adopted in 3GPP (3rd Generation Partnership Project) typically attempt to approximate beamforming with perfect or average (spatial) channel state information in the transmitter. Then, one obtains array gains as the signal transmitted by the different antenna elements (forming an array) can be combined coherently. Naturally, conveying the channel state information, or some function of channel state information (such as beam coefficients that maximize Signal-Interference-Noise-Ratio SINR for the given channel, supportable signalling formats, or the like) with sufficient reliability and transmission rate can require a (feedback) signalling channel of relative high capacity from the user equipment (UE) to the network and/or to the base station(s).
Note that the user equipment UE according to 3GPP and/or UMTS (Universal Mobile Telecommunication Standard) corresponds to a mobile station according to GSM (Global Standard of Mobile Communication). The present invention as described herein below is, however, not limited to be applied to a specific communication standard. References to UMTS 3GPP standard currently being developed serve as a mere example only and are not limiting the scope of the invention. The invention is related to any transmitting unit and any receiving unit. While the text below considers mostly downlink direction, it is evident that the roles of UE and network, i.e. Node_B or base station BS, may be exchanged here, i.e. the method of invention is equally applicable in uplink direction, or in any peer-to-peer link. Any transceiver device as described herein comprises a transmitter and a receiver. In a Node_B, the receiver receives feedback signals transmitted from the transmitter of a user equipment UE; and in a user equipment, the receiver receives downlink transmission signals transmitted from a transmitter of a Node_B.
FIG. 1 shows in rough outline the feedback control arrangement for closed-loop diversity concepts. A communication network NW 1 (e.g. UMTS or GSM or any other network) is represented by one of a plurality of Node_B's 2 (UMTS) (corresponding to a base station BS in GSM). The Node_B 2 is provided with an array of M antennas or antenna elements Ant1, . . . , Antm, . . . AntM from which a corresponding plurality of M beams beam_1, . . . , beam_m, . . . , beam_M emerges in direction to the user equipment UE 3. Each individual beam m (typically, 1<=m<=M) in downlink direction travels and/or propagates via a transmission channel 5 that can be represented by its channel transfer function hm. The array of antennas thus leads to an (M-dimensional) matrix of channel transfer functions H=[h1, . . . , hm, . . . , hM] representing the transmission channel between the Node_B 2 as a first transceiver and the intended receiver as a second transceiver, which may have one or more than one antenna elements. In particular, in MIMO channels the receiver 3 typically has more than one receiving antenna. For the purpose of the present invention, with regard to a downlink arrangement, a Node_B 2 and/or base station represents a first transceiver, while a user equipment UE 3 and/or mobile station represents a second transceiver. Notwithstanding this, as mentioned above, in case of an uplink arrangement, a Node_B 2 and/or base station represents a second transceiver, while a user equipment UE 3 and/or mobile station represents a first transceiver. However, the invention is not restricted to this but can be applied for any similar multi-antenna transmitter-receiver concept, either in downlink or in uplink direction.
Upon receiving a plurality of at least two beams from the first transceiver via the plurality of transmission channels at the second transceiver, the second transceiver performs well known processing techniques to e.g. obtain the channel transfer functions (estimates), and derives control information for being fed back from said second transceiver UE 3 to said first transceiver Node_B 2 in order to accomplish a feedback control. Note that the channel transfer function estimates may, for example, be obtained using Viterbi detection and/or processing of pilot signals containing known training sequences, using joint data detection, channel decoding, and channel estimation methods, or by any other suitable known method. For example, the channel estimates may be estimated based on CPICH measurements (CPICH=Common Pilot Channel).
Also, since a second transceiver, mostly for practical purposes, has a limited diversity capability as compared to the first transceiver (e.g. due to a limited space for plural antenna elements and due to cost reasons involved in designing the UE's/MS's representing in the chosen downlink example the second transceiver) the feedback path 4 carries beamforming information (control signals) related to Nbeam beams and the formation of Nbeam beams using a linear combination of M antenna elements and/or channels or beams. Based on the feedback beamforming information, an improvement of the performance of a subsequent transmission from the first transceiver to the second transceiver can be accomplished. Typically, the number of Nbeam satisfies the criterion of 1<=Nbeam<=M.
W. Utschnik et al presented in the Proc. 4th European Personal Mobile Communications Conference, Vienna, Austria, February 2000, a contribution entitled “Efficient tracking and feedback of DL-Eigenbeams in WCDMA” by means of which a distributed implementation of the eigenspace/-beam tracking at the user equipment and the base station, respectively, was proposed with a corresponding feedback signalling. The adopted feedback signalling is fixed.
In the 3GPP TSG RAN WG 1 Meeting #14, 4-7th July 200, Oulu, Finland, Siemens submitted a proposal for an “Advanced closed loop Tx diversity concept (eigenbeamformer)” for discussion. The feedback signalling used in the thus proposed arrangement is still considerable.
WO-A1-03/023995 also conceived by the present inventor and assigned to the present applicant discloses a closed-loop signalling method for controlling multiple transmit beams, which adopts a method that is improved in terms of its feedback signalling, thereby minimising an overhead due to feedback signalling. A fixed feedback capacity is sharedly used for transmitting beamforming information of Nbeam beams, with the partitioning of the feedback capacity depending on the dominance/importance of the respective information.
However, those previously proposed concepts are based on an assumption that the first transceiver uses the feedback control information as such. Furthermore, the second transceiver operates as a stand-alone entity when deriving feedback information.
Therefore, concepts described above are likely to be affected by errors which may occur or be introduced in the feedback control information during transmission, thus degrading the performance of the diversity or MIMO transmission. Also, the feedback transmission path still consumes a considerable amount of transmission resources/capacity.